In video system applications, a picture is displayed on a television or computer screen by scanning an electrical signal horizontally across the screen one line at a time. The amplitude of the signal at any one point on the line represents the brightness level at that point on the screen. A video frame contains the necessary information from all the lines that make up the picture and from all the associated synchronization (sync) signals to allow a scanning circuit to trace the lines from left to right and from top to bottom in order to recreate the picture on the screen. This information includes the luma (Y), or brightness, and the chroma (C), or color, components of the picture. There may be two different types of picture scanning in a video system. The scanning may be interlaced or it may be non-interlaced or progressive. Interlaced scanning occurs when each frame is divided into two separate sub-pictures or fields. The interlaced picture may be produced by first scanning the horizontal lines that correspond to the first field and then retracing to the top of the screen and scanning the horizontal lines that correspond to the second field. The progressive or non-interlaced picture may be produced by scanning all of the horizontal lines of a frame in one pass from the top to the bottom of the screen.
In video processing, the luma (Y) and chroma (C) signal components are modulated together in order to generate a composite video signal. Integrating the luma and chroma video elements into a composite video stream facilitates video signal processing since a single composite video stream is broadcasted. Once a composite signal is received, the luma and chroma signal components must be separated in order for the video signal to be processed and displayed. A comb filter may be utilized for separating the chroma and luma video signal components. For example, a television set may be adapted to receive a composite video input, but the chroma and luma video components have to be separated before the television can display the received video signal.
FIG. 1A is a diagram illustrating the generation of a conventional composite video signal. Referring to FIG. 1A, a conventional composite video signal 105 may be generated from a luma component 103 and a chroma component 101. The composite video signal 105 may be generated by adding the chroma video signal component 101 and the luma video signal component 103. The chroma signal component 101 may or may not comprise a constant chroma across the entire line. The luma signal component 103 may increase in amplitude in a stair step fashion or it may not.
FIG. 1B is a graph illustrating the frequency spectrum for a luma signal component 110 and a chroma signal component 112 of the composite video signal of FIG. 1A.
FIG. 2A is a diagram illustrating modulated chroma signals in contiguous composite video frames. The chroma component may be modulated so that a frequency of each successive line of video may be phase-shifted by 180 degrees with respect to the previous line. Referring to FIG. 2A, the previous frame 201 may comprise a previous line 203, a current line 205, and a next line 207. Similarly, the current frame 209 may comprise a previous line 211, a current line 213, and a next line 215. The current line 213 in the current frame 209 may be phase-shifted by 180 degrees from the previous line 211 in the current frame 209, as well as from the next line 215 in the current frame 209. Similarly, the current line 205 in the previous frame 201 may be phase-shifted by 180 degrees from the previous line 203 in the previous frame 201, as well as from the next line 207 in the previous frame 201. In addition, since fields in the contiguous composite video signal are at a frequency rate of 59.94 Hz for a NTSC signal, and 50 HZ for a PAL signal, there may be a 180-degree phase shift between two adjacent frames. For example, there may be a 180-degree phase shift between the current frame 209 and the previous frame 201. Correspondingly, the current line 213 in the current frame may be 180 degrees phase-shifted from the current line 205 in the previous frame 201.
In conventional video processing, there are three ways to separate the luma and chroma video components and these include combing horizontally, combing vertically, and combing temporally. During separation of the luma and chroma components, there are three bandwidth directions that may incur losses in the separation process and in the separated signal. Depending on the combing method that is utilized, the separated signal may have reduced vertical bandwidth, horizontal bandwidth, and/or temporal bandwidth.
The first way to separate the luma and chroma video components is by horizontal combing. Horizontal combing may be accomplished by utilizing a notch filter, such as a notch filter set at 3.58 MHz. Combing vertically may also be utilized to separate the luma and chroma video components. Combing vertically may be achieved in three different ways—the current line may be combed with the previous and the next line, the current line may be combed with the line just before it, or the current line may be combed with the line just after it. The vertical combing is performed spatially, which involves combing within one field at a time and without any temporal combing.
During combing in the current frame 209, for example, if the current line 213 is added to the previous line 211, the chroma content may cancel out and two times the luma content may be obtained. On the other hand, if the previous line 211 is subtracted from the current line 213, the luma content may cancel out and two times the chroma content may be obtained. In this way, luma and chroma content may be separated from the composite video signal for further processing. However, vertical combing may result in a reduced vertical bandwidth.
A third way to comb a composite signal is to comb temporally. Combing temporally comprises combing between two adjacent frames, for example, the current frame 209 and the previous frame 201. Further, temporal combing may be characterized by a reduced temporal bandwidth. Luma and chroma content may be separated by utilizing the same addition and subtraction method between a current line and a previous line as it was utilized with vertical combing.
FIG. 2B is a diagram illustrating combing of a correlated current line 224 and a previous line 222 in a current frame 220. In this case, there is no vertical bandwidth and the previous line 222 and the current line 224 are perfectly correlated. The current line 224 may be added with the previous line 222 and two times luma may be obtained. Similarly, the previous line 222 may be subtracted from the current line 224 so that two times chroma may be obtained.
FIG. 3 is a diagram illustrating combing of a non-correlated current line 334 and a previous line 332 in a current frame 330. In this case, there may be significant vertical bandwidth. The vertical bandwidth may be high enough so that there may be no correlation between the current line 334 and the previous line 332. When the current line 334 and the previous line 332 are combed together, there may be significant error in both the luma and chroma. This may produce combing artifacts in the obtained combed video signal. A substantially the same result may be obtained when combing temporally when there is temporal bandwidth, which indicates motion. Higher bandwidth in a given direction may cause combing in that direction to result in more incorrectly separated luma and chroma.
While 2D comb filters are adapted to process successive scan lines for a single field of a video frame, 3D comb filters are adapted to process scan lines that are taken from successive video frames. In general, for 3D comb filtering, if there is motion between the successive video frames, a 3D comb filter must revert to 2D comb filtering. Motion includes color changes and image movement between frames. Accordingly, the 3D comb filter may be required to buffer at least one frame in order to determine whether there is motion between the buffered frames. In instance where there is color changes or image movements between the buffered frames, the corresponding Y/C components for the buffered frames will be different and the results of combing would be incorrect.
Since the 3D comb filter may be required to buffer at least one frame of video data, several complete frames of video data must be stored in buffers as opposed to just 2 or 3 lines which are required by 2D comb filters. Accordingly, 3D comb filters require a large or significant amount of video memory and excessive memory processing bandwidth requirements. This large memory and excessive memory processing bandwidth requirements, along with the necessary motion detection processing, increases the cost associated with 3D comb filter solutions.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.